It has been suggested that a computer is a thermodynamic engine that sucks entropy out of data, turns that entropy into heat, and dumps the heat into the environment. The ability of prior art thermal management technology to get that waste heat out of semiconductor circuits and into the environment, at a reasonable cost, limits the density and clock speed of electronic systems.
A typical characteristic of heat transfer devices for electronic systems is that the atmosphere is the final heat sink of choice. Air cooling gives manufacturers access to the broadest market of applications. Another typical characteristic of heat transfer devices for electronics today is that the semiconductor chip thermally contacts a passive aluminum spreader plate, which conducts the heat from the chip to one of several types of fins; these fins convect heat to the atmosphere with natural or forced convection.
As the power to be dissipated by semiconductor devices increases with time, a problem arises: over time the thermal conductivity of the available materials becomes too low to conduct the heat from the semiconductor device to the fins with an acceptably low temperature drop. The thermal power density emerging from the semiconductor devices will be so high that even copper or silver spreader plates will not be adequate.
One technology that has proven beneficial is the heat pipe. A heat pipe includes a sealed envelope that defines an internal chamber containing a capillary wick and a working fluid capable of having both a liquid phase and a vapor phase within a desired range of operating temperatures. When one portion of the chamber is exposed to relatively high temperature it functions as an evaporator section. The working fluid is vaporized in the evaporator section causing a slight pressure increase which forces the vapor to a relatively lower temperature section of the chamber, defined as a condenser section. The vapor is condensed in the condenser section and returns through the capillary wick to the evaporator section by capillary pumping action. Because a heat pipe operates on the principle of phase changes rather than on the principles of conduction or convection, a heat pipe is theoretically capable of transferring heat at a much higher rate than conventional heat transfer systems. Consequently, heat pipes have been utilized to cool various types of high heat-producing apparatus, such as electronic equipment (see, e.g., U.S. Pat. Nos. 5,884,693, 5,890,371, and 6,076,595).
In some cases it is desirable for the heat pipe to be flexible, either to allow for thermal expansion (e.g. where the heat pipe has one or more bends to move around system components), or to provide vibration damping or insulation for the heat source. Often it is desirable to place the condenser in a remote location, either to provide access to forced cooling elements or to route the condenser to a space having a relatively low ambient temperature compared to that in which the evaporator is located. In some cases, the condenser is located near vibrating system components, and the condenser can pick up some of this vibration. With rigid heat pipes, this vibration can be transmitted back to the evaporator and thus to the component that is being cooled, such as a computer CPU.
One example of a flexible heat pipe is provided in U.S. Pat. No. 5,413,167 to Hara et al., in which one or more flexible heat pipes are used to provide heat transmission between a heat source and a heat exchanger. The Hara patent discloses a flexible heat pipe having a corrugated form to provide a desired flexibility. The wick is adhered to the interior surface of the bellows.
It would be advantageous to combine a bellows arrangement with a cable artery-type wick, rather than simply applying the wick to the interior surface of the bellows. This is because applying the wick material to the interior surface of the bellows corrugations limits the amount the bellows can be compressed. Thus, for very small size heat pipes there will be insufficient room for wick material between the corrugations while still allowing the desired compression. There are also issues of fragility of the bellows, change in stiffness (perhaps exceeding vibration transmissibility), and the extended length of travel for the condensate being wicked along the bellows surface (thus degrading wick maximum power capacity), all of which make application of wick material to the interior surface of the bellows undesirable. A cable artery-type wick, however, may not have the desired degree of axial flexibility due to the nature of its construction, and therefore when its ends are fixed to the evaporator and the condenser, it can form an undesirable rigid link between the two. Thus, there is a need for a flexible heat pipe system that combines the advantages of a bellows type heat pipe with a cable artery-type wick and also provides a desired degree of axial and lateral flexibility.